d
If the potential energy of the system is minimum, it will be stable equilibrium, i.e, $\frac{f^{2} U}{d x^{2}}>0$ and
when potential energy is maximum then it will be unstable equilibrium, i.e, $\frac{d^{2} U}{d x^{2}}<0$
As along y direction no electric field, potential energy is minimum and it will be stable
equilibrium along y-axis. Along $\mathrm{x}$ -axis potential energy is maximum due to all charges situated along $\mathrm{x}$ -axis.so it will be
unstable equilibrium.